Interlude

Wow! We’ve come a long way over the last eleven chapters. Seemingly ages ago, we started with some key fundamentals: wires and machines. We touched on function tables for describing how machines work, before moving to universal machines. We used our knowledge of symbol manipulation to come up with a representation of numbers, and then used those to learn how to add numbers. We set our sights on bigger things, and learned about multiwires and their annotations. With these more-powerful tools, we constructed polymorphic machines that operated on multiwires, and saw how polymorphic annotations could be manipulated with algebra. Most recently, we built a memory block, capable of selectively storing and retrieving information.

Throughout our journey, there has been a slow-but-steady trend: we’ve continuously been building bigger tools out of patterns of smaller ones. As we build these bigger and bigger tools, we’re capable of in turn using them to build bigger ones – tools that are too big to even consider building with less-sophisticated tools.

My friends, this is the process of abstraction. We’ve built amazingly capable things, having started from only two concepts – that we have wires and machines – and some rules how for piecing them together works. We defended this choice of an initial simplicity by way of parsimony, and today that choice is going to pay off in spades for us.

However, as we’ve seen, our notation of drawing these machines in terms of machine diagrams is clunky, and feels rather ham-fisted. Machine diagrams were amazingly helpful when we started, and it was easy to see exactly how everything fit together. However, now that multiwires are in the picture (if you’ll forgive the pun), and now that our diagrams are getting quite large, it’s hard to keep everything in our mind at once. It seems desirable that we come up with a new representation for these logical systems we have been building, so that we can continue abstracting (our semantics) without worrying too much about the form it comes in (our syntax).

And so, today, we’re going to look at an alternative representation of wires and machines, one that will fit our purposes better. We’ll build in a notion of multiwires from the get-go, so that things end up looking “prettier” when we need to use them later. But before we do any of these things, we need to discuss types.

Types

Types are a powerful abstraction on top of what we’ve been calling multiwire annotations. Types are what multiwire annotations secretly strive to become – what they stay up at night researching how to get a promotion. Or something like that.

Quick! Pop quiz! Name as many colors as you can. Maybe you came up with things like red, orange, and mauve, among others. It might not feel like it, but this exercise you just performed highlights an interesting phenomenon.

Ok, now name four things you might purchase at McDonalds. Done? Great.

This is one of those “obvious-not-obvious” Mr. Bruce questions I mentioned earlier. What is it, you might ask? It’s “how can we perform such an exercise?” How come “french fries” might have been an answer to “what can you buy at McDonalds”, but “hiking boots” assuredly wasn’t.

An even deeper question is: “what kinds of distinctions are we drawing in order to answer this?”

The answer to this Mr. Bruce question is that the distinctions we’re drawing is one between groups and members. The groups are the “categories” of things we’re looking for, and the members are the things that belong in that group. While we might not be able to name all of the things in the group, it’s usually pretty easy to come up with a few examples. And because the universe only has so many atoms in it, there must be a limit on the total number of things in these groups.

And if there is a total number, then we can count to it if we were really dedicated. And if we can count to something, it must be countable.

In order to determine whether something is a member of a group or not, we need a notion of being able to distinguish that thing. We need to be able to identify it. This sounds obvious, but it’s worth stating, lest we dive off into crazy territory.

We call these groups types, and we call their members values. We add an additional constraint, that any value has exactly one type it is a member of. This sounds unnecessarily restrictive at first blush, but, as we’ll see, it doesn’t stop us from doing anything we’d like, and it makes the reasoning much easier.

Before you dive in and start conjuring up types and values like there’s no tomorrow, let’s slow down and pretend like types don’t exist until we’ve defined them together.

The most obvious type given our history is the type of a wire, with values 0 and 1. We even called them values to begin with, so that’s a good sign. This type is strictly more interesting than just being for wires, so we’ll call it Bool. This silly sounding name is actually short for a sillier sounding one: “Boolean,” which means “of or pertaining to George Boole.” Mr. Boole was the guy who came up with the idea of a type with only two values in it, so we name it in his honor.

Our notation for defining types is thusly:

type Bool = On | Off

which should be read as “the type Boolis defined as being either On or Off”, the vertical bar meaning “or.”

This definition tell us something, not only a relationship about what Bool is, but also it tells us that On is a Bool, and Off is also a Bool. Obvious stuff really, but, again, worth stating explicitly. We write these facts as On : Bool and Off : Bool, where the : symbol should be read as “is a value and has the type of.”

You should be reminded by this notation of our multiwire annotations. And indeed, our multiwire annotations in the form of A:n can be interpreted in the same way: A is a value and has the type of n.

Values describe things that are, while types describe things that could be. If I tell you to give me an On, there is exactly one value which that might be. On the other hand, if I tell you to give me a Bool, you’re free to pick any value of type Bool.

Takeaway: Values are things that are. Types are things that could be.

If we think of a wire being equivalent to type Bool, then it would make sense that the possible values of a wire are On and Off. Our old annotation :2 was equivalent to having two wires side-by-side, and when interpreted as a type, it must then be a combination of two Bools.

The annotation :2 corresponds to the type (Bool, Bool), which you can think of as a pair of two Bools, sitting side-by-side exactly like their wire counterparts were. But what are the values of this type (Bool, Bool)? As you might expect, it’s all of the combination of two wires. We’ll list them here:

(Off, Off) : (Bool, Bool)

(Off, On ) : (Bool, Bool)

(On, On ) : (Bool, Bool)

(On, Off) : (Bool, Bool)

Notice that we have four values, not three as some might expect. (On, Off) is distinct from (Off, On), because it actually matters which wire is on and which is off.

For the time being, however, we’ll restrict ourselves to primarily thinking about the Bool type.

A New Foundation

With types under our belt, we can now look at our new representation of machine “diagrams”. Recall that the simple and gate could equivalently be described as a picture:

or, alternatively, as a function table:

Input A

Input B

Output

0

0

0

0

1

0

1

1

1

1

0

0

We chose to use the picture more often than not, because it was easy to keep track of how the inputs and outputs connected. Well, it was easy at first, at least!

Our new representation of machines instead celebrates the function table. Here’s how we’d represent the and gate in our new formalization:

and : Bool -> Bool -> Bool
and Off Off = Off
and Off On = Off
and On On = On
and On Off = Off

As you can see, this closely mirrors the function table. Except for the first line, which is obviously something else. But what is it? Well, that first line describes the interface of the machine!

The trick to reading the interface line is to look at the very last type (after all of the arrows), and say “aha! This is the output of the machine!” All the other types should be considered as inputs.

and : Bool -> Bool -> Bool should thus be interpreted as a machine which takes two Bools and outputs one Bool. Exactly like our old and gate showed:

The syntax for describing these machine interfaces initially seems… well, let’s admit it: suspicious at best. However, there is a really really good explanation for why, which we’ll discuss in a few chapters. It looks a lot like how we describe the type of a value, so as a good rule of thumb “if there are any arrows after the :, then it is a machine we’re discussing.” Otherwise, it’s a value with a type annotation.

Now that we know how to make machines in our new foundation, the only thing left is to connect machines to one another, but that’s a discussion for the next chapter.

Exercises

Define the machine in our new notation with the interface nand : Bool -> Bool -> Bool. Given the fact that we can make this machine, how much of the thing we’ve built so far with machine diagrams can we safely assume will transfer over to the new representation?